RHODONITE Chapter17. Inosilicates

RHODONITE

(Mn,Ca)SiO₃
Triclinic, P1, a = 6.68, b = 7.66, c = 12.20 Å,
a = 111.1, b = 86.0, g = 93.2^o, Z = 10




	Figure 17-21. Crystal drawings of rhodonite from Franklin. These are three sets of two projections (A and B each) of three individual crystals. Drawings are from Palache (1935) who provided crystallographic data.

Rhodonite is one of the more colorful and abundant silicate minerals in the Franklin orebody. It is also known, in lesser quality, from Sterling Hill. Local material has been much studied. Early analyses of impure material were given by Torrey (1822), Fowler (1825), and Thomson (1828), who referred to it as manganesian feldspar, siliceous oxide of manganese, and ferro-silicate of manganese, respectively. The keatingine of Shepard (1876) is rhodonite or bustamite. The varietal term which endured is fowlerite, named by Nuttall in the 1820’s for Dr. Samuel Fowler (Shepard, 1832a). See Palache (1935) for an historical review.




	Figure 17-22. Crystal drawings of rhodonite from Franklin. These are very typical habits. Drawings are from Palache (1935) who provided crystallographic data.

Franklin rhodonite was studied by Gossner and Brükl (1928); they showed that it was not isostructural with diopside and provided crystallographic data. Subsequent crystallographic studies were by Hilmer et al. (1956) and by Liebau et al. (1959), whose data are cited above. Viswanathan and Harneit (1986) noted that highly zincian rhodonites have large beta-angles. The literature is replete with references to the relations of rhodonite to other pyroxenoids and different choices of unit-cell; the most accepted of these is that of Ohashi and Finger (1975). The choice of unit-cell is reviewed by Deer et al. (1978). An X-ray powder pattern is given by the ICDD, PDF #13-138.

Description

The habit and forms of Franklin rhodonite were neatly outlined by Pirsson (1890); others were published by Ford and Crawford (1911). A summary was exhaustively set out by Palache (1935) with many crystal drawings (Figures 17-21 and 17-22) and photographs.

In general, the preponderance of the available crystals, obtained by removal from calcite matrix, are square to rectangular in cross-section, many with resorbed crystal edges (Figure 17-25). Rhodonite also occurs as irregular segregations, platy masses, fine-grained material, vein fillings, and foliated masses.

Franklin rhodonite is pink, red, brownish red, and gray; Sterling Hill rhodonite is generally pale grayish pink, but bright pink material is known. The color has been the subject of much comment. Manning (1968) attributed it to Mn²⁺ based on absorption spectra in the visible and near-ultraviolet regions. A spectrographic interpretation of shock-induced color changes in Franklin rhodonite led Gibbons et al. (1974) to propose that the color was due to small amounts of Mn³⁺. This interpretation was not accepted by Faye (1975) who presented an argument against the presence of Mn³⁺; this argument, however, was not accepted by Gibbons et al. (1975). Frondel (1972) has noted that some pink rhodonite may fade on exposure to light.

Franklin rhodonite has a vitreous luster and perfect and good cleavages. The density varies from 3.4 to 3.68 g/cm³. Optical data for Ca-rich crystals which underly marsturite (Dunn and Leavens, 1986), as given by Larsen and Shannon (1922b), are: biaxial, positive, large 2V, with a = 1.716, b = 1.720, and g = 1.732; dispersion is perceptible and crossed. There is no discernible fluorescence in ultraviolet. Rhodonite is distinguished from bustamite by its positive optic sign and indices of refraction and from pyroxmangite by its lower birefringence and larger 2V. The gem potential of Franklin rhodonite was discussed by Webster (1975).

Composition

Rhodonite is a manganese silicate of the pyroxenoid group, the low-temperature form of MnSiO₃, and locally always contains some calcium and zinc. Numerous analyses of Franklin rhodonites have been obtained by the writer; some are given in Table 11. Solid solution of Fe and Mg is generally limited in Franklin specimens; however, some Sterling Hill specimens contain relatively large amounts of iron. Samples intimately associated with tremolite have average compositions of FeO 1.5, MgO 1.0, and CaO 7.5 wt. %. The maximum calcium content found locally is 23 mole % CaSiO₃. None of the analyzed samples has a Zn content quite as high as a few of those reported by Palache (1935); the maximum Zn content found by this writer is 7.6 wt. % ZnO or 12 mole % ZnSiO₃.




	Table 11. Chemical analyses of rhodonite.

The Franklin specimens with the highest Zn content are the bright pink euhedral crystals in calcite, many up to 5 cm in size, which were figured by Palache (1935). Conversely, the rhodonites with the lowest Zn contents are those associated with large masses of apparently recrystallized bright green willemite. The Sterling Hill specimens with the highest zinc contents are dull, prismatic, brownish pink or grayish pink crystals and masses. Unlike zincian bustamite, which may have exsolved willemite, no willemite exsolution was seen in any of many hundreds of rhodonite specimens examined in a deliberate search for this relation. Willemite has in some cases formed within incipient cleavage planes in rhodonite, especially in sheared material, and this relation gives a false impression of exsolution.

Some compositional relations are evident from numerous chemical analyses, but these are general and qualified. Rhodonite crystals from primary assemblages at Franklin and Sterling Hill have much more solid solution of Mg, Fe, and Zn, for Mn than the secondary crystals found in vein assemblages, which are presumably of lower temperature and are depleted in these contaminant elements; secondary crystals have an enrichment of Ca and Mn (Figure 17-32). Rhodonite phase equilibria were given by Peacor et al. (1978b), and thermodynamic data were given by Bennington et al. (1987).

Occurrence and paragenesis




	Figure 17-23. Crystals of rhodonite from Franklin. Field of view is 10 cm in maximum dimension. Smithsonian Institution, #C2484. Photo by Vic Krantz.

The occurrences of rhodonite at Franklin are very numerous. It is associated with many of the major calcium silicate minerals and occurs as both coarse-grained and fine-grained assemblages. Ries and Bowen (1922) noted it occurring in abundance with andradite at the borders of pegmatites (Figures 12-28 and 12-29) and locally replacing willemite. It was repeatedly reported at the orebody-marble contacts, and Palache (1935) noted its occurrences in the Trotter Mine and the Parker Mine.




	Figure 17-24. Rectangular prismatic crystal of rhodonite, associated with andradite, from a vein assemblage at Franklin. Field of view is 1.2 mm in maximum dimension.

The premier occurrence for Franklin rhodonite specimens is the assemblage of euhedral, bright pink rhodonite crystals in white calcite with franklinite and minor willemite. Such crystals may occur in large sizes; some are 19 cm, and many are 1-7 cm. These are the crystals illustrated by Palache and others. Also known are elongate grayish-pink pseudoprismatic composite crystals, up to 10 x 2 x 2 cm, with coarse surfaces, some in subparallel growth, and embedded in gray calcite with franklinite and minor willemite (Figures 17-30, 17-33, and 17-34).




	Figure 17-25. Stout rhodonite crystals in calcite (white) with minor franklinite crystals (black at bottom) from Franklin. Specimen is 3 cm in maximum dimension. Privately owned. Photo by the author.

Rhodonite is also found in thin, bladed crystals, most commonly in seams; the crystals are commonly composite, and some exhibit parallel growth. They are inconsistently associated with prismatic, 1 mm willemite crystals and colorless to white, 1-2 cm barite crystals. This assemblage may be host to cahnite and/or hedyphane.

A notable Franklin assemblage is that originally described as rhodonite with bustamite by Larsen and Shannon (1922b) and subsequently described as rhodonite with marsturite by Dunn and Leavens (1986). In this vein assemblage, rhodonite occurs as bright pinkish-red crystals, with one lustrous pinacoid roughly parallel to the surface of the specimen (Figure 17-17). The rhodonite crystals are coated on other forms by marsturite. The assemblage consists of rhodonite with marsturite, manganaxinite (Figures 16-16 and 16-17), and radial sprays of ganophyllite (Figure 18-36).




	Figure 17-26. Bright pink rhodonite crystals in white calcite with franklinite from Franklin. Specimen is 6 cm in maximum dimension. Privately owned. Photo by the author.		Figure 17-27. Blocky, near-rectangular crystals of rhodonite (gray) with calcite (white) in a vug in ore composed of franklinite (black), rhodonite, and calcite from Franklin. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #116813. Photo by the author.




	Figure 17-28. Superb rhodonite crystals with white calcite from Franklin. Specimen is 5 cm in maximum dimension. Smithsonian Institution, #B5746. Photo by the author.		Figure 17-29. Blocky crystals of rhodonite from Franklin. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #C2481-21. Photo by the author.

In addition to these well-crystallized assemblages, rhodonite also occurs as massive aggregates in the calcium-silicate units of the orebody, associated with a large number of minerals, among them andradite, mica (some is hendricksite), actinolite, tremolite, feldspar, andradite, quartz, fluorite, microcline, franklinite, gahnite (Figure 22-41 and 22-42), sphalerite, and many others. Coarsely-crystallized intergrowths with brown andradite and green willemite are particularly attractive.




	Figure 17-30. Atypical rhodonite crystals in calcite from Franklin. Specimen is 15 cm in maximum dimension. Mineralogical Museum, Harvard University, #122151. Photo by the author.		Figure 17-31. Pseudo-prismatic crystals of rhodonite in calcite (white) with franklinite (black) from Franklin. Specimen is 6 cm in maximum dimension. Smithsonian Institution, #136936. Photo by the author.

Rhodonite is found associated with bustamite in several unstudied and uncommon assemblages. In one, rhodonite with composition (Mn_0.64Ca_0.22Zn_0.09Mg_0.03Fe_0.02)SiO₃ occurs as an apparent thin-skinned crystal-mold, 1 x 3 cm in cross section, which is filled as a pseudomorph with a microcrystalline bustamite with composition (Mn_0.51Ca_0.36Zn_0.08Mg_0.02Fe_0.03)SiO₃ (#C2482-1). Others include bustamite between franklinite/calcite ore and rhodonite (#R14460) and the reverse relation (#R3115-1) (Figure 17-9). Some rhodonite in ore is surrounded by a tephroite-willemite symplectite.




	Figure 17-32. A cluster of thin bladed rhodonite crystals from Franklin. Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #105602. Photo by Chip Clark.		Figure 17-33. Large elongate rhodonite crystals with uncommon habit associated with franklinite (black) and minor calcite (white) from Franklin. Specimen is 27 cm in maximum dimension. Smithsonian Institution, #C42. Photo by the author.

Rhodonite is also common as a vein mineral, occurring with willemite, calcite, and other minerals and forming veins which vary in color from bright pinkish red to dull brownish pink; the latter occurs massive and is a host for hedyphane. These are called “ribbons” by some local collectors, in allusion to the appearance of the pink-colored band in otherwise common ore.




	Figure 17-34. Cluster of large elongate crystals of rhodonite with calcite on franklinite-willemite ore from Franklin. Specimen is 17 cm in maximum dimension. Mineralogical Museum, Harvard University, #122150. Photo by Chip Clark.

Palache (1935) mentioned a material referred to as hydrorhodonite, which was reported as an alteration product of rhodonite. Examination of one specimen so labeled in the collections at the National Museum found it to be massive, fine-grained, and nearly amorphous to X-rays. After long exposure to X-radiation, it yielded a diffuse pattern, slightly similar to that of tephroite. Palache (1935) also noted the alteration of rhodonite to serpentine, in whole or in part, but this has not been investigated in detail. Thin, transparent “films” on rhodonite crystals are commonly serpentine; they may provide false lusters, both dull and waxy. The now-obscure name dyssnite (Thomson, 1828) and a variant dyshnite (Fowler, 1825) have been applied to such alteration products. Rhodonite weathers to a black color.




	Figure 17-35. Bands of granular rhodonite (gray) and franklinite (black) from Franklin. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #144595. Photo by the author.

Rhodonite is less common at Sterling Hill and not commonly found in the bright pink color of many Franklin samples; it is pale grayish pink or brownish pink. Some Sterling Hill rhodonite secondary veins, however, are quite pink. Metsger et al. (1958) reported rhodonite occurring as a 1-inch thick zone between a franklinite-pyroxene zone and the ore of the brown-willemite areas. See also Jenkins (1994). Sterling Hill material is varied in its associations; among many observed are:

a)    veins of brownish rhodonite with friedelite and willemite in franklinite/willemite ore.

b)    shear zones in varied assemblages, containing light pink rhodonite, associated on various specimens with tremolite, fluorite, garnet, clinopyroxene, willemite, and calcite.

c)    massive gray-pink rhodonite, some forming large crystals associated with mica, calcite, and gahnite, between the 700 and 1000 levels, in the east branch of the west limb.

d)    coarse-textured assemblages, including tephroite-rhodonite-calcite, and numerous other minerals associated on various specimens with calcite, clinopyroxene (likely diopside), andradite, and franklinite.

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